This invention generally relates to an improved method of electrocoating water-soluble or water-dispersible coating resins onto a conductive surface and specifically to the control of amines in an amine neutralized polycarboxylic acid resin system, such as is taught in U.S. Pat. Nos. 3,230,162; 3,382,165; and the like.
Electrodeposition of water-based coatings, commonly termed electrocoating, is a coating process which has many advantages over other coating methods, such as spraying, dipping, rolling and the like. The advantages are numerous. The process deposits a film of uniform thickness on essentially any conductive surface even those having sharp points and edges. The electrocoated film applied is relatively water-free, and thus will not run or drip when taken out of the bath. Because little or no organic solvents are used in resin systems, the process is essentially fumeless and requires no extensive fume collection and incineration equipment. This latter point is important in view of the increased concern over environmental pollution. An additional advantage is the fact that a second or top coat can be applied over the electrocoated film without curing the electrocoated film and both coats can be cured in one baking operation. By eliminating the necessity of two furnaces, the cost of a two-coat process can be considerably reduced.
The electrocoating process generally comprises immersing the article to be coated into the electrocoating bath, usually as an anode, and passing a current through the bath between the article and an electrode. The process is usually self-arresting in that as the thickness of the coating increases, the resistance thereof also increases, thereby decreasing the electrodeposition rate.
Most commercially available resins for anodic electrocoating generally are polycarboxylic acid-based resins, such as those described in the previously mentioned patents which issued to Gilchrist. To solubilize the resins, usually the resin is completely or nearly completely neutralized by a base, such as an organic amine, such as diisopropanolamine, dimethylethanolamine, triethylamine or ammonium hydroxide or other suitable base, such as KOH, LiOH and NaOH. During electrocoating when the polycarboxylic acid-based resin is electrodeposited, the organic amine or other neutralizing agent is released taking on a hydrogen ion and is driven to the cathode where H.sub.2 is usually liberated. The organic amine or other neutralizing agent is not deposited in the coating and will stay in the bath except for small amounts which may be lost through dragout. Unless removed, the neutralizing agent will gradually build up in the bath and interfere with the electrocoating process. To remove the neutralizing agent and other low molecular materials, the bath is usually passed through an ultrafilter or other suitable device. For an excellent discussion on the use of ultrafilters in the purification of an electrocoating bath, see the article "Ultrafiltration of Electrocoating Systems," in Nonpolluting Coatings and Coating Processes, Plenum Press, (1973) edited by J. L. Gardon and J. W. Prane and the article written by A. S. Michaels in Chemical Engineering Progress, Vol. 64, No. 12, 31-43.
Prior methods of controlling the neutralizing agent and other components in the electrocoating bath generally comprise determining the concentration of neutralizing agent and other components in the bath and then discarding a particular amount of the ultrafilter permeate for those components in excess, such as organic amines, and adding components, such as solubilizing or coupling agents and deionized water in which the bath is deficient. Usually, deionized water must be added due to the water which is discarded with the permeate. However, unless the analysis and permeate disposal is done frequently, a considerable variation in the amine level and other components can result, thus seriously interfering with the electrocoating operations. Frequent analysis is both troublesome and expensive.
Against this background, the present invention was developed.